Method for manufacturing ceramic substrate, ceramic substrate, and silver-based conductor material

ABSTRACT

A method for manufacturing a ceramic substrate containing glass includes a firing step in which an unfired silver-based conductor material is disposed on an unfired ceramic layer and is fired. The unfired silver-based conductor material contains at least one of a metal boride and a metal silicide.

TECHNICAL FIELD

The present invention relates to a method for manufacturing a ceramicsubstrate, a ceramic substrate, and a silver-based conductor material.

BACKGROUND ART

There has been known a multi-layer ceramic substrate fired at a lowtemperature, which is also called a low temperature co-fired ceramic(LTCC) substrate. Such an LTCC substrate is usually manufactured bylaminating a plurality of green sheets, each having a wiring traceformed of an unfired conductor material, and firing the green sheets(for example, see the following Patent Documents 1 and 2, etc.).

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Patent Application Laid-Open (kokai) No.H6-252524

Patent Document 2: Japanese Patent Application Laid-Open (kokai) No.2007-234537

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

A process of manufacturing a ceramic substrate using a silver-basedconductor material, not limited to the above-mentioned LTCC substrate,has a problem of diffusion of silver contained in the conductor materialinto the ceramic during firing. This may cause formation of voids in thesubstrate, deformation of the substrate, and change of the color of thesubstrate. Conventionally, there have been proposed techniques of addingvarious substances to the silver-based conductor material so as toprevent diffusion of silver during firing. For example, Patent Document1 discloses a technique of coating the surfaces of the particles of asilver-based conductor powder with an antimony salt or an antimonatesalt. Patent Document 2 discloses a technique of adding a silicon (Si)powder to a conductor paste.

However, even when such a substance is added to the silver-basedconductor material, the effect of preventing silver diffusion cannot beattained to a sufficient degree in the case where a reaction caused bythe added substance to prevent diffusion of silver does not occur asexpected at temperatures near the firing temperature. Thus, there isstill a room for improvement regarding prevention of diffusion of silverduring firing in the process of manufacturing such a ceramic substrate.

Means For Solving the Problem

The present invention has been accomplished to solve at least theabove-described problem by employing a method different fromconventional ones. The present invention can be realized as thefollowing modes.

[1] One mode of the present invention is a method for manufacturing aceramic substrate containing glass. The manufacturing method includes afiring step. The firing step may be a step of firing an unfired ceramiclayer and an unfired silver-based conductor material disposed on theunfired ceramic layer. The unfired silver-based conductor material maycontain at least one of a metal boride and a metal silicide. Themanufacturing method of this mode prevents diffusion of silver duringfiring because at least one of a metal boride and a metal silicide isadded in the unfired silver-based conductor material. The unfiredsilver-based conductor material may be disposed on a surface of anunfired ceramic layer, between unfired ceramic layers adjacent to eachother, or in through holes formed in an unfired ceramic layer.

[2] In the manufacturing method of the above-mentioned mode, the metalboride may be at least one of lanthanum hexaboride, silicon hexaboride,titanium diboride, and tantalum diboride. The manufacturing method ofthis mode prevents diffusion of silver during firing more effectively.

[3] In the manufacturing method of the above-mentioned mode, the metalsilicide may be at least one of titanium disilicide, zirconiumdisilicide, tungsten disilicide, chromium disilicide, molybdenumdisilicide, and tantalum disilicide. The manufacturing method of thismode prevents diffusion of silver during firing more effectively.

[4] In the manufacturing method of the above-mentioned mode, the unfiredsilver-based conductor material contains the metal boride or the metalsilicide. The amount of the metal boride or the metal silicide withrespect to the amount of the inorganic components of the unfiredsilver-based conductor material may be greater than 3 vol. % and lessthan 20 vol. %. The manufacturing method of this mode prevents diffusionof silver during firing more effectively and also prevents impuritiesfrom remaining in the conductor of the substrate.

[5] In the manufacturing method of the above-mentioned mode, the unfiredsilver-based conductor material contains a silver powder and at leastone of the metal boride and the metal silicide may be attached tosurfaces of particles of the silver powder in the silver-based conductormaterial. The manufacturing method of this mode prevents oxidation ofsilver during firing more effectively. As a result, the effect ofpreventing diffusion of silver into a ceramic layer improves.

[6] A second mode of the present invention is a ceramic substrate. Theceramic substrate may include a ceramic layer and a wiring layer of asliver-based conductor which are formed by the firing step according toany one of the manufacturing methods of the above-mentioned mode. Theceramic substrate of this mode prevents problems such as formation ofvoids in the ceramic substrate, warpage of the ceramic substrate, changeof the color of the ceramic substrate, etc.

[7] A third mode of the present invention is a silver-based conductormaterial which is unfired and fired together with an unfired ceramiclayer to form a wiring layer in a ceramic substrate. The silver-basedconductor material of this mode may contain at least one of a metalboride and a metal silicide. The silver-based conductor material of thismode prevents diffusion of silver in a process of manufacturing aceramic substrate. In the silver-based conductor material of this mode,the metal boride may be at least one of lanthanum hexaboride, siliconhexaboride, titanium diboride, and tantalum diboride. In thesilver-based conductor material of this mode, the metal silicide may beat least one of titanium disilicide, zirconium disilicide, tungstendisilicide, chromium disilicide, molybdenum disilicide, and tantalumdisilicide.

All the plurality of constituent elements of each mode of the presentinvention are not essential. In order to solve, partially or entirely,the above-mentioned problem or yield, partially or entirely, the effectsdescribed in the present specification, a part of the elements may beproperly modified, deleted, or replaced with another new element, or thelimitation thereof may be partially removed. Also, in order to solve,partially or entirely, the above-mentioned problem or yield, partiallyor entirely, the effects described in the present specification, aportion or all of the above-described technical features contained inone mode of the present invention may be combined with a portion or allof the above-described technical features contained in other modes ofthe present invention to thereby attain an independent mode of thepresent invention.

The present invention can be realized as various modes other than amethod for manufacturing a ceramic substrate, a ceramic substrate, or asilver-based conductor material. For example, the present invention canbe realized as a method for firing a ceramic substrate, a method formanufacturing a silver-based conductor material, an apparatus forimplementing those methods, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Schematic view showing a structure of an LTCC substrate.

FIG. 2 Flowchart showing steps of a process of manufacturing the LTCCsubstrate.

FIG. 3 Explanatory view showing the results of an experiment forchecking an effect of preventing diffusion of silver by adding anadditive to a conductor paste.

FIG. 4 Explanatory views showing scanning electron microscope (SEM)images of LTCC substrates and images showing the silver concentrationdistribution profile in each of the LTCC substrates.

MODES FOR CARRYING OUT THE INVENTION A. Embodiment

FIG. 1 is a schematic view showing the structure of an LTCC substrate 10according to one embodiment of the present invention. The LTCC substrate10 which is a ceramic substrate is used, for example, for electroniccomponents, high-frequency modules, IC packages, or printed wiringboards used in computers, communication devices, etc. The LTCC substrate10 has a multi-layer structure formed by laminating a plurality ofceramic insulating layers 11. Each of the ceramic insulating layers 11is formed by low temperature firing whose firing temperature is 1000° C.or lower.

Each of the ceramic insulating layers 11 has vias which are throughholes for disposing via electrodes 12. The LTCC substrate 10 has wiringlayers including internal electrodes 13 and external electrodes 14, eachformed between ceramic insulating layers 11 adjacent to each other. Thewiring layers are electrically connected to one another through the viaelectrodes 12 formed in the ceramic insulating layers 11.

In the LTCC substrate 10 of the present embodiment, each of theelectrodes 12 to 14 is formed of a silver-based conductor material whosemain component is silver. In this description, a “main component” meansa material component which accounts for at least 50 mass % of themixture. On the outermost surface of the LTCC substrate 10 are disposedpassive elements (resistors, etc.) and active elements (ICs, etc.) whichare connected to the external electrodes 14. In this description,illustration and detailed description of the passive elements and theactive elements are omitted.

FIG. 2 is a flowchart showing steps of a process of manufacturing theLTCC substrate 10. The LTCC substrate 10 is manufactured by firing anunfired ceramic material (green sheet) and an unfired silver-basedconductor material together at low temperature.

In step 1, a green sheet which constitutes an unfired ceramic layercontaining ceramic particles and glass particles is prepared. The greensheet is made by preparing a ceramic slurry by mixing together inorganiccomponents (including a glass powder and an inorganic filler), a bindercomponent, a plasticizer, and a solvent, and forming the ceramic slurryinto the shape of a sheet using the doctor blade method or the like.

In step 2, a conductor paste which forms electrodes 12 to 14 and whichis an unfired silver-based conductor material is prepared. The conductorpaste is made by mixing together a powder of the silver-based materialand a glass powder which are inorganic components, and an organicsolvent and a resin which is a varnish component.

Notably, the inventor of the present invention has found that adding atleast one of a metal boride and a metal silicide into the conductorpaste as an inorganic component prevents diffusion of silver containedin the conductor paste, or a silver component of the conductor paste,into the ceramic insulating layer during a firing step described later.It is considered that oxygen present near the conductor paste isconsumed by oxidation of the metal boride or the metal silicide duringthe firing step, whereby oxidation of the silver contained in theconductor paste is prevented.

In step 2 of the present embodiment, an additive including at least oneof a metal boride and a metal silicide is added to the conductor paste.For example, the following substances can be used as an additive to beadded to the conductor paste.

Examples of the metal boride include lanthanum hexaboride (LaB₆),silicon hexaboride (SiB₆), titanium diboride (TiB₂), tantalum diboride(TaB₂), niobium diboride (NbB₂), chromium diboride (CrB₂), molybdenumboride (MoB), zirconium diboride (ZrB₂), tungsten boride (WB), vanadiumdiboride (VB₂), and hafnium diboride (HfB₂). Examples of the metalsilicide include zirconium disilicide (ZrSi₂), titanium disilicide(TiSi₂), tungsten disilicide (WSi₂), molybdenum disilicide (MoSi₂),tantalum disilicide (TaSi₂), chromium disilicide (CrSi₂), niobiumdisilicide (NbSi₂), iron disilicide (FeSi₂), and hafnium disilicide(HfSi₂).

The metal borides and the metal silicides described above are justexamples. The additive may be a metal boride or a metal silicide otherthan those described above. However, the metal boride or the metalsilicide used as an additive is preferably a one which initiates areaction with oxygen during the firing step described later. Inparticular, the metal boride or the metal silicide preferably has anoxidation temperature which is lower than the firing temperature duringthe firing step 4 described later. The “oxidation temperature” is a peaktemperature at which oxidation occurs, and is a value measured throughthermogravimetric-differential thermal analysis (TG-DTA). Specifically,the oxidation temperature of the metal boride or the metal silicide asan additive is preferably 800° C. or lower, and more preferably 700° C.or lower. Also, the oxidation temperature of the metal boride or themetal silicide as an additive is preferably 400° C. or higher, and morepreferably 500° C. or higher. Diffusion of silver is prevented if theoxidized silver is not wetted by the glass material contained in thegreen sheet when the glass material softens during the firing step. Forthis reason, the oxidation temperature of the metal boride or the metalsilicide as an additive is preferably lower than the glass-transitiontemperature of the glass material contained in the green sheet preparedin step 1.

The additive may be added in the form of powder, for example,concurrently with or after the step of mixing the inorganic componentsand the varnish components. Alternatively, the additive may be addedbefore mixing the inorganic components and the varnish components. Inthis case, the additive is added in such a manner that the surfaces ofthe particles of the silver-based material contained in the inorganiccomponents are coated with the additive. For example, the silver-basedmaterial can be coated with the additive by the following method. First,the additive is dissolved or dispersed in an organic solvent (toluene,xylene, or alcohol). Then, a powder of the silver-based material isdispersed or suspended in the solution or dispersion of the additive.The solvent is kept still for a predetermined time or stirred so as tocause the additive to adhere to the surfaces of the particles of thesilver-based material. Coating the silver-based material with theadditive as described prevents oxidation of silver to a greater degree,and improves the effect for suppressing silver diffusion. The additivemay be added to the conductor paste using a method other than thatdescribed above.

The amount of additive with respect to the amount of the inorganiccomponents of the conductor paste is preferably more than 3 vol. %, morepreferably more than 5 vol. %. This condition allows the effect forsuppressing silver diffusion to be attained more reliably. The amount ofadditive with respect to the amount of inorganic components of theconductor paste is preferably less than 20 vol. %, more preferably lessthan 18 vol. %. This condition prevents impurities originating from theadditive in the conductor paste from remaining in the LTCC substrate 10after firing.

In step 3, the above-described conductor paste is disposed on the greensheet. Specifically, vias are formed in the green sheet by a hole-makingoperation such as punching, and the vias are filled with the conductorpaste. A wiring trace is printed on each surface of the green sheet byapplying the conductor paste thereto by means of screen printing or thelike. After the wiring trace is formed, a plurality of such green sheetsare laminated to form an unfired laminate.

In step 4, the unfired laminate is fired at a low temperature. Thefiring temperature in step 4 may be a temperature preset in accordancewith the glass-transition temperature of the material component of thegreen sheet prepared in step 1. Specifically, the firing temperature instep 4 may be, for example, approximately 750° C. to 950° C. After step4, the LTCC substrate 10 is completed. Passive elements and activeelements to be connected to the electrodes 14 are disposed on thecompleted LTCC substrate 10.

As described above, in the LTCC substrate 10 of the present embodiment,addition of the metal boride or the metal silicide to the conductorpaste in step 2 prevents diffusion of silver from the silver-basedconductor material to the ceramic insulating layer 11. This preventsdeterioration of electrical insulation of the ceramic insulating layers11 caused by diffusion of silver. Local change of the color of theceramic caused by a change in the composition of the ceramic near thewiring trace as well as local deterioration of the strength of theceramic insulating layer 11 are also prevented. In addition,acceleration of firing-caused contraction only near the conductor pasteis prevented, and formation of voids between the electrodes 12 to 14 andthe ceramic insulating layers 11 are prevented.

FIG. 3 is an explanatory view showing the results of an experiment forchecking an effect of preventing diffusion of silver by adding theadditive to the conductor paste. This experiment checked diffusion ofsilver into the ceramic insulating layers using samples S01 to S18(missing numbers: S04 and S16) of LTCC substrates manufactured throughuse of a conductor paste containing an additive and samples T01 to T03of the LTCC substrates manufactured through use of a conductor pastecontaining no additive. The specific conditions for manufacturingsamples S01 to S18 (missing numbers: S04 and S16) and T01 to T03 are asfollows.

<Compositions of the Green Sheets>

For samples S01 to S03, S05 to S12, S18, T01, and T03, green sheetscontaining an SiO₂—B₂O₃—CaO glass and alumina (Al₂O₃) were prepared. Forsamples S13 to S15, S17, and T02, green sheets containing anSiO₂—CaO—BaO—MgO glass and alumina (Al₂O₃) were prepared.

<Procedure For Preparing the Green sheets>

(1) A powder of borosilicate-based glass whose main components aresilica (SiO₂), alumina (Al₂O₃), and boric acid (H₃BO₃) and a powder ofalumina were put into a pot formed of alumina such that their volumeratio became 60:40 and the total weight became 1 kg.

(2) Subsequently, 120 g of acrylic resin and proper amounts of methylethyl ketone (MEK) serving as a solvent and dioctyl phthalate (DOP)serving as a plasticizer were put into the pot formed of alumina. Theamounts of methyl ethyl ketone and dioctyl phthalate were determinedsuch that the desired levels of slurry viscosity and sheet strengthcould be attained.

(3) The materials mentioned above were mixed for five hours to therebyobtain a ceramic slurry.

(4) A green sheet with a thickness of 0.15 mm was made from the ceramicslurry using the doctor blade method.

<Conductor Paste>

(1) Conductor pastes for samples S01 to S17 (missing numbers: S04 andS16)

A mixture of the following inorganic components, varnish components, andadditive was kneaded with a triple roll mill, whereby the conductorpastes for samples S01 to S17 were prepared.

Inorganic components: a silver powder and a borosilicate glass powder

Varnish components: ethyl cellulose resin and terpineol solvent

Additive: any one of LaB₆, SiB₆, TiB₂, TaB₂, ZrSi₂, TiSi₂, WSi₂, CrSi₂,MoSi₂, and TaSi₂

The amount of additive with respect to the amount of the inorganiccomponents of the conductor paste was set to 15 vol. % for samples S01to S03, S05 to S10, and S13 to S15, 9 vol. % for samples S11 and S17,and 3 vol. % for sample S12. The oxidation temperatures shown in thetable were measured through the TG-DTA method.

(2) Conductor Paste For Sample S18

After the surfaces of the particles of the silver powder which is aninorganic component were coated with SiB₆ which is an additive, themixture of the above-described inorganic components and varnishcomponents was kneaded with a triple roll mill, whereby the conductorpaste for sample S18 was prepared. The amount of additive with respectto the amount of the conductor paste were 15 vol. %.

(3) Conductor Pastes For Samples T01 to T03

The conductor pastes for samples T01 and T02 were prepared by the samemethod as that for samples S01 to S17 (missing numbers: S04 and S16)except that no additive was added. The conductor paste for sample T03was prepared by the same method as that for samples S01 to S17 (missingnumbers: S04 and S16) except that in place of the metal boride or themetal silicide, SiO₂ was added as an additive.

<Forming and Firing an Unfired Laminate>

(1) Vias were formed in the green sheet and filled with the conductorpaste. A wiring trace was formed on a surface of the green sheet byapplying the conductor paste thereto. A plurality of such green sheetswith the wiring trace formed thereon were laminated to form an unfiredlaminate.

(2) Unfired laminates for samples S01 to S18 (missing numbers: S04 andS16) and T01 to T03 were fired. The firing temperature for samples S01to S03, S05 to S12, S18, T01, and T03, which used SiO₂—B₂O₃—CaO greensheets, was set to about 850° C. The firing temperature for samples S13to S15, S17, and T02, which used SiO₂—CaO—BaO—MgO green sheets, was setto about 900° C. The firing time was set to approximately 60 minutes forall samples S01 to S18 (missing numbers: S04 and S16) and T01 to T03.

The “silver diffusion distance” shown in FIG. 3 will be described withreference to FIG. 4. Each of sections (A) to (G) of FIG. 4 shows ascanning electron microscope (SEM) image of a cross section of the LTCCsubstrate parallel to a direction of lamination of the LTCC substrateand an image of the same cross section as the SEM image captured by anelectron probe micro analyzer (EPMA). In each section of FIG. 4, the SEMimage is shown on the upper side and the image captured by EPMA is shownon the lower side. The image captured by an EPMA (hereinafter, simplyreferred to as the “EPMA image”) shows the silver concentrationdistribution profile of the LTCC substrate in colors in response to thelevel of silver concentration. Sections (A) to (F) of FIG. 4 show theSEM images and the EPMA images of samples S02, S03, and S05 to S08 whichwere manufactured through use of different conductor pastes containingSiB₆, TiB₂, ZrSi₂, TiSi₂, WSi₂, and CrSi₂, respectively, as an additive.Section (G) of FIG. 4 has the SEM image and the EPMA image of sample T01which was manufactured through use of a conductor paste containing noadditive. In the center of each SEM image and each EPMA image, theinternal electrode formed of the silver-based conductor extends in thehorizontal direction of the images. The EPMA image shown in section (G)of FIG. 4 shows that silver diffused into a wide region extending in thevertical direction of the image from the internal electrode such thatthe silver concentration in the region is approximately the same as thatin the internal electrode. The inventors of the present inventionacquired the SEM images and the EPMA images of predetermined polishedcross sections of samples S01 to S18 (missing numbers: S04 and S16), andT01 to T03. The inventor determined a “silver diffusion distance” foreach sample from the EPMA image of each sample. Specifically, theinventor used the concentration of Ag at an electrode interface throughwhich the internal electrode is in contact with the ceramic insulatinglayer as a reference concentration, and measured, at five points, thedistance from the electrode interface to a region in which theconcentration of Ag in the ceramic insulating layer becomes equal to orless than half the reference concentration. The average of the measureddistances was used as the “silver diffusion distance.”

The silver diffusion distances were 30 μm or less in all samples S01 toS18 (missing numbers: S04 and S16) which were manufactured through useof the conductor paste containing the metal silicide or the metal borideas an additive. By contrast, the silver diffusion distances were greaterthan 30 μm in samples T01 to T03 which were manufactured without use ofthe conductor paste containing the metal silicide or the metal boride asan additive. These results show that the metal silicide or the metalboride added to the conductor paste prevented diffusion of silver fromthe conductor material during firing.

If the same additive was added to the conductor paste, diffusion ofsilver was prevented approximately to the same degree (see samples S01to S03 and samples S13 to S15, and samples S11 and S17) irrespective ofthe composition of the green sheet. The test results show that diffusionof silver was prevented to a great degree in both the case where theadditive was added to the conductor paste in the form of powder and thecase where the additive was added to the conductor paste as a materialfor coating the surfaces of the silver powder particles (see samples S01and S18).

Particularly, the silver diffusion distance was restrained to a valuesmaller than 5 μm in any of samples S01 to S03, S05, S10 to S15, S17,and S18 in which one of LaB₆, SiB₆, TiB₂, TaSi₂, and ZrSi₂ was added tothe conductor paste as an additive in an amount greater than 3 vol. %.It should be noted when SiB₆ is used as an additive, SiO₂ generated byoxidation during firing remains in the ceramic insulating layer. Thatis, in the case where SiB₆ is used as an additive as in sample S02, onlya compound of the same composition as the compound contained in theceramic insulating layer remains in the ceramic insulating layer. As aresult, migration of impurities into the ceramic insulating layer isprevented.

As described above, in the manufacturing process (FIG. 2) of the presentembodiment, the metal boride or the metal silicide added to theconductor paste prevents diffusion of silver from the conductor materialduring the firing step. Accordingly, the LTCC substrate 10 manufacturedaccording to the manufacturing process can prevent various types ofproblems caused by diffusion of silver from the conductor materialduring the firing step such as formation of voids in the ceramicsubstrate, deterioration of the ceramic substrate, etc.

B. Modifications B1. Modification 1

In the above-described embodiment, a single type of metal boride or asingle type of metal silicide is added to the conductor paste as anadditive. However, both a metal boride and a metal silicide may be addedto the conductor paste as additives. A plurality of types of metalborides may be added in combination as additives. A plurality of typesof metal silicides may be added in combination as additives.Alternatively, one or more types of metal borides may be added togetherwith one or more types of metal silicides as additives.

B2. Modification 2

In the above-described embodiment, in the process of manufacturing theLTCC substrate, at least one of a metal boride and a metal silicide isadded to the conductor paste which is a silver-based conductor material.However, in a process of manufacturing a ceramic substrate other thanthe LTCC substrate, the additive described above may be added to asilver-based conductor material. For example, in a process ofmanufacturing a ceramic substrate whose firing temperature is 1000° C.or higher, the additive described above may be added. The silver-basedconductor material containing at least one of a metal boride and a metalsilicide added thereto is not required be in the form of paste, but maybe, for example, in the form of powder.

B3. Modification 3

In the above-described embodiment, in preparation of the green sheet,alumina is used as an inorganic filler. However, as an inorganic fillerused for preparation of the green sheet, a material other than aluminamay be used. As an inorganic filler, for example, mullite can be used.

The present invention is not limited to the above-described embodiment,examples, and modifications, but may be embodied in various other formswithout departing from the spirit of the invention. For example, inorder to solve, partially or entirely, the above-mentioned problem oryield, partially or entirely, the above-mentioned effects, technicalfeatures of the embodiments, examples, and modifications correspondingto technical features of the modes described in the section “SUMMARY OFTHE INVENTION” can be replaced or combined as appropriate. Also, thetechnical feature(s) may be eliminated as appropriate unless the presentspecification mentions that the technical feature(s) is mandatory.

DESCRIPTION OF REFERENCE NUMERALS

-   10 . . . LTCC substrate-   11 . . . ceramic insulating layer-   12 . . . via electrode-   13 . . . internal electrode-   14 . . . external electrode

What is claimed is:
 1. A method for manufacturing a ceramic substratecontaining glass, comprising a firing step of firing an unfired ceramiclayer and an unfired silver-based conductor material disposed on theunfired ceramic layer, wherein the unfired silver-based conductormaterial contains a metal boride.
 2. The manufacturing method accordingto claim 1, wherein the metal boride is at least one of lanthanumhexaboride, silicon hexaboride, titanium diboride, and tantalumdiboride.
 3. (Canceled)
 4. The manufacturing method according to claim1, wherein the unfired silver-based conductor material contains themetal boride and the amount of the metal boride with respect to theamount of the inorganic components of the unfired silver-based conductormaterial is greater than 3 vol. % and less than 20 vol. %.
 5. Themanufacturing method according to claim 1, wherein the unfiredsilver-based conductor material contains a silver powder and the metalboride is attached to surfaces of particles of the silver powder in thesilver-based conductor material.
 6. A ceramic substrate comprising aceramic layer and a wiring layer of a sliver-based conductor which areformed by the firing step according to claim
 1. 7. A silver-basedconductor material which is unfired and is fired together with anunfired ceramic layer to form a wiring layer in a ceramic substrate,wherein the unfired silver-based conductor material contains a metalboride.
 8. A method for manufacturing a ceramic substrate containingglass, comprising a firing step of firing an unfired ceramic layer andparticles of an unfired silver-based conductor material disposed on theunfired ceramic layer, wherein the particles of the unfired silver-basedconductor material are coated with at least one of a metal boride and ametal silicide.